The problem with the browsers is that the standards are not comprehensible by any human being.

If you put a random MIT-graduate in a room with the standards and nothing else, there is no chance that they would be able to answer a set of questions as to how a bunch of different documents would have to be rendered. Anyone building a new browser would look at the output of existing browsers to figure out what is supposed to happen.

The programming languages used and the apparent complete lack of engineering practices in these browsers is further evidence that they don't want any outsider involved.

Compiling Firefox wasn't that difficult a decade ago.

The Linux kernel is open enough to find out why some things don't work, but it's not open enough to understand why every line is there, which often results from hardware vendors not documenting their hardware, so you can never prove that the driver code is correct.